A spring contact pin for contacting an electrical or electronic circuit article to be tested such as a circuit board or an integrated circuit for test purposes is generally electrically conductive and can comprise a sleeve in which one shank of a front contact bolt is guided slidably and projects from the front end of the sleeve, a contact head serving to contact the item or the device to be tested being provided on the end of the shank of the front contact bolt located outside the sleeve and at least one inner spring within the sleeve and acting on the front contact bolt to spring load the contact bolt in the sleeve.
Spring contact pins are known from and described in Kruger: "Prufmittel zur Elektrischen Prufung von Leiterplatten fur Uhren", Jahrbuch der Deutschen Gesellschaft fur Chronometrie, Band 30, 1979, S. 269(Testing Means for Electrical Testing of Circuit Boards for Watches, Yearbook of the German Chronometry Society, 30, pp. 269(1979). They each have a sleeve in which a coil spring is mounted which spring loads a straight contact bolt which projects from the sleeve exteriorly. This contact bolt has a shank or stem slidably mounted in the sleeve on which a contact head is mounted and which is intended to come into contact with the item to be tested.
The electrical or electronic circuit article to be tested can include a circuit board, an integrated circuit such as a chip or the like. A testing unit to test such an circuit article to be tested can have a test adapter which is connected to an electrical testing unit or analyzer or computer.
The test adapter chiefly has a plurality of spring contact pins arranged in a predetermined grid or other array in one or more plates parallel to each other and next to each other. The number of spring contact pins of a test adapter can, if necessary, be very large. Most test adapters have hundreds or thousands, often even many thousands of spring contact pins.
Each spring contact pin must exert a comparatively large force (contact force) with its contact head on the circuit article to be tested so that a reliable electrical contact occurs and oxidation layers, dirt layers or the like can be penetrated by the contact head at the position at which the circuit article is to be tested.
The required contact force in the known spring contact pin commonly used in practice is exerted by a single coil spring located in the sleeve and amounts mostly to more than 100 cN, often several hundred cN, but in many cases somewhat less than 100 cN. The spring exerting the contact force often must endure an extremely large number of load changes without breaking, e.g. millions of load changes, so that one should not load it to its extreme limit.
The spring contact pins must be mounted so as to be electrically insulated from each other side-by-side in the test adapter with very small spacing and may have only a very small maximum outer diameter so that they can be positioned on the test adapter in great density. The outer diameter of the circular cylindrical portion of the sleeve of the spring contact pin is usually from about 0.5 to 2.5 mm, but in many cases can be still larger or smaller.
The maximum outer diameter of the contact head mounted on the shank of the contact bolt should not exceed that of the sleeve if possible to avoid electrical contact between the contact heads of the close-spaced spring contact pins which are adjacent each other.
Also the length of the spring contact pin may be relatively small although it will be comparatively large in relation to the diameter of the sleeve. For example the length of the spring contact pins in the unloaded state is chiefly in a size range of about 1 to 12 cm.